Antennas: shock danger

Discussion in 'Wireless & RF Design' started by msr, Feb 21, 2010.

  1. msr

    Thread Starter Active Member

    Jul 8, 2008
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    Hello,

    Some time ago I wrote here a question about antenna's shock danger. I forgot I had made that post here, so Im writing a new one. I apologise for that.

    I don't even know RF burns exists. Can an antenna create a RF burn somewhat like a tesla coil?

    So, the shock danger depends on the antenna power right?

    From what power it could be dangerous?
    Most "consumer" antennas haven't such power right?

    For example: tv antennas and monopole antennas we see in some buildings/infrastructures

    Thanks in advance for you attention!
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Papabravo

    Expert

    Feb 24, 2006
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    An antenna connected to a receiver is not dangerous, unless it gets hit by lightning while you are holding on to it. The lightning danger is not restricted to antennas, any conductor will do just fine.

    Antennas connected to transmitters is another matter. I would treat any transmitting antenna as "hot" in much the same way that I treat every propeller as if it were moving.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    YES, if you can touch it and are suitably grounded you can experience significant current flow. AKA Shocked. :-O

    The RF is dangerous mostly when it becomes a source of 'ionization' inside your bodies wet meaty parts. The higher the frequency being transmitted along with the amount of power being radiated determines at what distance, if any, that the energy will become ionizing. Meaning it will disprupt and destroy living tissue at that distance and closer. Aboard my ship there were several low frequency 'flagpole' antenna the danger line around those was 7 feet in radius. On the other hand, locking our illuminating radar onto groups of seagulls several miles away would cause them to panic and scatter. :)
     
    Last edited: Feb 21, 2010
  5. msr

    Thread Starter Active Member

    Jul 8, 2008
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    A nice way to bring more fish home in deed! :D

    Thank you all for the responses!
    In conclusion: you can touch your TV antenna but not something like this http://deepspace.jpl.nasa.gov/dsn/images/picture_70_bw_lg.jpg. The transmitters are the dangerous ones.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Anyone who's been around high power broadcasting for a while knows about R.F. burns....and they HURT. And the worst part is, you don't need a complete "circuit" to get one. The capacitance of your body is enough to act as the second terminal. R.F. burns at A.M. broadcast frequencies are the most painful...it's low enough in frequency that the skin effect is rather small...so they burn DEEP. VHF is nowhere near the problem, as skin effect is very high...so penetration is very little.


    Eric
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
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    I believe animal bodies can be modeled as a resistor in parallel with a capacitance. Direct current sees only the body's inherent resistance, which is our skin's resistance. Dry, it's in the 100s of kilohms. Wet, it's much higher, especially when wet means sweat, which includes salt and hence forms an electrolyte.

    But AC takes advantage of the skin's inherent capacitance and here is the formula for it's impedance (resistance): X=1/(2*pi*f*C). Notice that as f, the frequency of the AC current, increases, X gets smaller. And I, the current, equals the voltage divided by the impedance (I=V/X). Therefore the current increases with frequency because X gets smaller with increased frequency. And it is the electrical current that burns or even kills an animal.
     
    Last edited: Feb 23, 2010
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    I can attest that 15,750 Hz give burns that leave lines of black char under the skin.

    This is correct in a situation where the voltage will also increase. It is good that antennas have a fixed voltage applied.

    The RF skin effect - as separate and utterly distinct from human skin - limits the penetration of the AC voltage as the frequency increases. The conduction path stays very near the surface in a broad swath, rather than in a narrow path where the current density produces charring.

    The thing to take home is that while you get hurt in different ways, you still get hurt if in contact with a charged conductor - DC, line voltage, or live transmitting antenna.
     
  9. PRS

    Well-Known Member

    Aug 24, 2008
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    Actually, I said the impedance varies (goes down) with increasing frequency and consequently the current goes up, becoming ever more lethal with increasing frequency.

    I may be mistaken but it seems to me the current penetrates deeper than the skin. The heart muscle gets constricted when the nerves controlling its muscles are charged and cardiac arrest ensues. If there was no penetration, I don't see how this would be possible.

    As for getting shocked harder with increasing f, try this: Using a function generator that keeps the amplitude of the signal flat while you turn up the frequency, turn down the frequency as low as it will go and the voltage very low and touch your tongue to the output leads. Turn up the voltage until you feel a slight tingle on your tongue. Then, leaving the voltage there, turn the freqency up slowly. You'll be forced to quit at about 50 hertze due to the pain. ;)
     
  10. beenthere

    Retired Moderator

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    I suppose we could go on at some further length as to an agreed-on definition of skin, and who can stand the most electrically caused discomfort.

    As an alternative, let me repeat a part of my last post -
     
  11. PRS

    Well-Known Member

    Aug 24, 2008
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    We certainly came across an interesting subject here. I understand the skin effect when it comes to metal wires, but I really don't see its application to human skin, there being a whole different set of physics involved for each.

    I think a lot of people, even professionals are confused when it comes to electrocution and burning by electric current. I don't pretend to be an authority on the subject but having noticed the variation of current penetration with frequency as noted above, I have come to realize there must be a capacitive effect at play here.

    Currently, I'm thinking the skin is forming a dialectric between two inductors -- the probe or wire carrying the current on the one hand and the blood and other tissue lying under the skin, which is a conductor on the other side of the dialectric. Say you grab two wires -- the hot in one hand, the ground in the other. The current travels through the epidermis due to ohms law -- voltage and skin resistance -- and due to capacitance when we have an ac current. Then it passes through the veins and body tissues to the the other hand and exits due to the same.

    The skin is 1/32 (eyelids) to 1/8 (heals) inch thick. The blood and tissues are essentailly an electrolyte as in a battery. This simple mechanism is probably complicated by a variety of resistances and capacitances throughout the body.

    More if you want. :)
     
  12. PRS

    Well-Known Member

    Aug 24, 2008
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  13. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    We had a ET (Navy) long ago grab a URT-23 PA tube under full RF power right in front of me. It burned an arc (2250VDC + 4000V RF) from the palm of his hand thru inside his arm and out the elbow where it was touching the side of the case. Lucky for him it knocked him out cold. We rushed him to sickbay were the doctors first were just going to cut his STILL smoking arm off but we had a surgeon onboard that needed work so they opened it up like a dead fish and scrapped all the ash out and then grafted parts and skin from all over his body. They did a good job because a year later he re-enlisted and was back to full duty.
    http://www.boatanchors.org/URT-23B.htm

    [​IMG]
     
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